The present invention relates generally to multilayer antireflection coatings for substrates, and more particularly to multilayer antireflection coatings deposited on transparent substrates by sputtering.
The simplest antireflection coating is a single layer of a transparent material having a refractive index less than that of a substrate on which it is disposed. The optical thickness of such a layer may be about one-quarter wavelength at a wavelength of about 520 nanometers (nm), at about the visible spectrum. The visible spectrum extends from a wavelength of about 420 nm to a wavelength of about 680 nm. A single layer coating produces a minimum reflection value at the wavelength at which the layer's optical thickness is one-quarter of the wavelength. At all other wavelengths the reflection is higher than the minimum but less than the reflection of an uncoated substrate. An uncoated glass surface having a refractive index of about 1.52 reflects about 4.3 percent of the normally-incident light. Coating the substrate with a layer of magnesium fluoride (MgF.sub.2) having a refractive index of about 1.38 produces a minimum reflection of about 1.3 percent.
Multilayer antireflection coatings are made by depositing two or more layers of transparent dielectric materials on a substrate. At least one layer has a refractive index higher than the refractive index of the substrate. The layer systems usually include at least three layers and are designed to reduce reflection at all wavelengths in the visible spectrum. Multilayer antireflection coatings may yield reflection values of less than 0.25 percent over the visible spectrum.
Most multilayer antireflection coatings are derived from a basic three layer system. The first or outermost layer of this system has a refractive index lower than that of the substrate and an optical thickness of about one-quarter wavelength at a wavelength of about 520 nm. The second or middle layer has a refractive index higher than that of the substrate and an optical thickness of about one-half wavelength at a wavelength of about 520 nm. The third layer, i.e. the layer deposited on the substrate, has a refractive index greater than that of the substrate but less than that of the second layer. The optical thickness of the third layer is also about one-quarter wavelength at a wavelength of about 520 nm. This basic design was first described in the paper by Lockhart and King, "Three Layered Reflection Reducing Coatings", J. Opt. Soc. Am. , Vol. 37, pp. 689-694 (1947).
A disadvantage of the Lockhart and King system is that the refractive indices of the layers must have specific values in order to produce optimum performance. The selection and control of the refractive index of the third layer is particularly important. Deviation from specific refractive index values can not be compensated for by varying the thickness of the layers.
Various modifications of the Lockhart and King system have been made to overcome these disadvantages. For example, the layer system has been modified by forming at least one layer from mixtures of two materials having refractive indices higher and lower than the desired value for the layer. The refractive index of one or more has also been simulated by using groups of thinner layers having about the same total optical thickness as the desired layer, but including layers having refractive index values higher and lower than the desired value.
Other modifications have included varying the refractive index of one or more of the layers as a function of thickness, having the refractive index of a layer inhomogeneous in the thickness direction. This approach is described in U.S. Pat. No. 3,960,441. Another modification is the use of an additional layer between the basic three layer system and the substrate. This additional layer may have an optical thickness of about one-half wavelength, i.e., about half the thickness of the basic system, and a refractive index less than that of the substrate. This modification is disclosed in U.S. Pat. No. 3,781,090.
The layer systems discussed above are generally deposited by thermal evaporation. In thermal evaporation, the time required to deposit the layers may be only a relatively small fraction of the total production time. The production time may be determined by such factors as pump down time for the coating chamber, the time required to heat substrates to process temperatures, and the time required to cool substrates after coating. The number of layers in the coating, the thickness of the layers, and the layer materials may not have a significant influence on production time and thus cost.
DC reactive sputtering is the process most often used for large area commercial coating applications. For example, this process may be carried out in a glass coater or in-line system to deposit thermal control coatings for architectural and automobile glazings. In the glass coater, the articles to be coated are passed through a series of in-line vacuum chambers, each including sputtering sources, i.e., sputtering cathodes. The chambers are isolated from one another by vacuum locks.
The time taken to deposit the layers is determined mainly by the number of layers and the sputtering rate of the materials. The use of a glass coater to deposit multilayer antireflection coatings can significantly reduce their cost, extending their range of application.
Many of the materials used in thermal evaporation processes, particularly fluorides and sulfides, are not easily sputtered. Conversely, a few materials, such as zinc oxide (ZnO), commonly used in the architectural glass sputtering systems are rarely, if ever used, in thermal evaporation processes. The sputtering rate of different materials may vary by a factor of greater than twenty. The choice of materials, therefore, can have a significant influence on the deposition time and fabrication cost. In an in-line sputtering system with multiple chambers, each chamber may be set up to deposit one specific material. As such, the number of layers that can be deposited is determined by the number of chambers. A coating designed for sputter deposition should therefore be as simple as possible. It should also be made, if possible, from materials which have a high sputtering rate.
A simple improvement on the Lockhart and King system, described in U.S. Pat. No. 3,432,225, the entire disclosure of which is hereby incorporated by reference. This system, called the Rock system, includes four layers. The first or outermost layer has a refractive index lower than that of the substrate and an optical thickness of about one-quarter wavelength at a wavelength of about 520 nm. The second or middle layer has a refractive index higher than that of the substrate and an optical thickness of about one-half to sixth-tenths of a wavelength at a wavelength of about 520 nm. The third layer has an optical thickness of about one-tenth of a wavelength at a wavelength of 520 nm and a refractive index less than that of the second layer. The fourth layer has an optical thickness of about one-tenth of a wavelength and a refractive index greater than the second layer and the substrate. The third layer may be the same material as the first layer, and the fourth layer may be the same material as the second layer.
Even though the Rock system is somewhat similar to the Lockhart and King system, in that it has about the same total optical thickness, it functions in a unique way. Specifically, for a selected set of materials, the layer thicknesses of the Rock system may be adjusted to provide optimum performance. Specific refractive index values for the layers are not required. However, in order to obtain a low reflection value, for example less than 0.5 percent across the entire visible spectrum, the refractive index of the first and third layers should be less than about 1.5, and the refractive index of the second and fourth layers should be greater than about 2.2. A Rock system suitable for sputtering may use silicon dioxide (SiO.sub.2) for the first and third layers, and titanium dioxide (TiO.sub.2) for the second and fourth layers.
The Rock system is simple as it has only four layers. However, since it requires a relatively high refractive index material, such as titanium dioxide, a high sputtering rate is difficult to obtain. Typically, the deposition rate for titanium dioxide reactively sputtered from titanium is only one-quarter that of silicon dioxide reactively sputtered from silicon. For a Rock system using titanium dioxide and silicon dioxide, the deposition of titanium oxide would take about four times longer than the deposition of silicon dioxide.
Certain materials with refractive indices less than about 2.2 have relatively high sputtering rates. For example, zirconium dioxide (ZrO.sub.2) can be deposited about twice as fast as titanium dioxide, tin oxide (SnO.sub.2) about ten times faster than titanium dioxide, and zinc oxide (ZnO) about fifteen times faster than titanium dioxide. Zirconium oxide has a refractive index of about 2.1, tin oxide of about 2.0 and zinc oxide of about 2.0.
The problem with using high sputtering rate materials for the Rock system is illustrated by FIG. 1. Curve A shows the reflection values, as a function of wavelength, for a system incorporating titanium dioxide and silicon dioxide layers. The layer sequence and thickness are shown in Table 1. The layer thickness is described as fractions of a wavelength at a wavelength .lambda..sub.O, which is known as the center wavelength or the design wavelength.
TABLE 1 ______________________________________ Refractive Optical Index Thickness Layer Material @510 nm .lambda..sub.0 = 510 nm ______________________________________ Air 1.0 Entrance Medium 1 SiO.sub.2 1.46 0.2148.lambda..sub.0 2 TiO.sub.2 2.35 0.5204.lambda..sub.0 3 SiO.sub.2 1.46 0.1022.lambda..sub.0 4 TiO.sub.2 2.35 0.0567.lambda..sub.0 Glass 1.52 Substrate ______________________________________
Curve B shows the reflection values for a system in which zinc oxide, instead of titanium oxide, is used for the second layer. The layer thicknesses were modified slightly to obtain optimum result with the new combination of materials. The layer sequence and thickness are shown in Table 2.
TABLE 2 ______________________________________ Refractive Optical Index Thickness Layer Material @510 nm .lambda..sub.0 = 510 nm ______________________________________ Air 1.0 Entrance Medium 1 SiO.sub.2 1.46 0.2369.lambda..sub.0 2 ZnO 2.01 0.3959.lambda..sub.0 3 SiO.sub.2 1.46 0.0771.lambda..sub.0 4 TiO.sub.2 2.35 0.0441.lambda..sub.0 Glass 1.52 Substrate ______________________________________
As shown, the structure of Table 2 provides antireflection performance (curve B) over a narrower spectral region than the structure of Table 1 (curve A). This is evident from the higher reflectivity at the extremes of the visible spectrum at about 420 nm (WB) and 680 nm (WR). The reflectivity of the Table 2 structure (curve B) in the spectral range from 460 to
640 nm is two to three times higher than the Table 1 structure (curve A). Thus, the higher production rate produced by using zinc oxide instead of titanium oxide is only accomplished at the expense of a significant reduction in performance.
It is object of the present invention to provide an antireflection system for economical, high volume production in an in-line reactive sputtering apparatus.
It is another object of the present invention to provide an antireflection coating with a performance comparable to the Rock system but wherein only about one-quarter of the total optical thickness of the coating includes a material having an index of refraction greater than about 2.2.
It is a further object of the present invention to provide an antireflection coatings which may use a material having a refractive index of about 2.0 for the thickest, high refractive index layer.
It is yet another object of the present invention to provide an antireflection coating wherein at least the material used to form the thickest, high refractive index layer has a sputtering rate about ten times faster than that of titanium dioxide.
It is still another object of the present invention to provide an antireflection coating which has no more than five layers and wherein the total optical thickness is less than or equal to about one wavelength at a wavelength between about 480 nm to 560 nm.